1. Field of the Invention
The present invention relates to solar cells designed for operation under concentrated sunlight, and more particularly to concentrator cells having optimum band gaps.
2. Description of the Prior Art
It is known that in order to generate electricity economically using sunlight and solar cells, one needs the following:
(1) low-cost solar collectors, and PA1 (2) high-efficiency energy converters.
The photovoltaic cell, commonly known as the solar cell, is an attempt to satisfy these two requirements with the same element. Alternatively, a low-cost lens can be utilized to concentrate the sunlight onto a small-area high-efficiency solar cell. The lens thus acts as the low-cost solar collector, in combination with a more expensive high-efficiency energy converter. Solar cells designed to operate with concentrated sunlight are a special class of solar cells distinctly different in many respects from the more conventional fiat-plate solar cells.
A concentrator solar cell is a solar cell operated at the focus of a lens or reflector system. As with any solar cell, high performance requires good junction quality and high minority-carrier diffusion lengths. However, a concentrator cell operates at higher light-generated current density than does a flat-plate cell. This higher current density operation allows for higher energy conversion efficiencies, provided the grid series resistance can be kept small. High-quality material is required in order to obtain these results. The semiconductor material used must have an acceptably low mid-gap recombination state density. If the solar cell is ideal, its performance is predictable from the semiconductor intrinsic energy gap. For materials with direct band gaps, all the incident light with photon energy above the band gap is absorbed, creating minority-carriers that diffuse to the junction where they are collected. This light-generated current is opposed by a much smaller dark current consisting of majority-carriers diffusing over the junction barrier. This junction barrier is again related primarily to the semiconductor intrinsic energy gap.
Doubling the light intensity incident on a solar cell in turn doubles the device short-circuit current. This in itself does not change the device energy-conversion efficiency. However, when the current in a diode is increased, the diode voltage increases. Since the solar cell voltage increases with increasing light levels, the result is that the solar cell energy-conversion efficiency increases when operated with concentrated light.
To date, the highest terrestrial efficiencies for single-junction solar cells, approximately 28%, have been realized with GaAs homojunction devices at concentration ratios of approximately 200 suns. Close behind are Si concentrator cells which have reached efficiencies of approximately 27% at similar concentration ratios.
Solar cells operating tinder concentrated sunlight in areas of high direct insolation have been identified as one of the more cost effective applications of photovoltaics. Previous efforts in the field have focused on the use of Si and GaAs cells in these systems. However, efficiency gains on the cell level have a dramatic impact on the system cost analysis. Consequently, a determination of the true optimum band gaps has not been pursued for these materials, but rather, more efficient alternatives to GaAs and Si, such as multi-junction concentrator devices, are being investigated. A disadvantage of multi-junction devices is that the fabrication of monolithic multi-junction cells is quite complex and thus potentially costly.
Consequently, there remains a need for a single-junction terrestrial device designed for operation under concentrated sunlight having higher energy-conversion efficiencies and reduced usage of costly photovoltaic (PV) materials.